In cold chamber die-casting machines it is known to use injection pistons with a steel or copper body and at least one outer sealing band arranged astride of a collar at the piston head.
An example of such pistons is described in U.S. Pat. No. 5,233,912.
EP1197279, by the same applicant, describes a piston for cold chamber die-casting machines which comprises a steel body having a head with or without peripheral chamfer and at least one copper alloy sealing band arranged about the body in a respective seat obtained in retracted position relative to the head, and wherein on the outer piston surface comprised between the head and the band there are obtained at least two channels intended for placing the piston head in communication with the annular seat of the band for metal inflow underneath the band itself. In this way, by solidifying, the metal that flows into the seat creates a continuous thickening that radially pushes the band outwards, progressively recovering the wear thereof, adapting it to any deformations of the piston container and therefore protecting the latter.
However, it has been found that in all the known embodiments thereof, the sealing band tends to rotate on the piston making the operation thereof less effective.
In fact, all sealing bands have a split or interruption shaped as a step along the circumference that allows assembling the band on the piston and ensures certain radial expansion thereof. Especially in the assembly step, when the band is released, the elasticity thereof could cause settling in an unsuitable position that impairs the sealing thereof. For example, the band split should not be:                facing the top side of the container, where fused aluminium exhibits greater fluidity (in fact, aluminium lays by gravity on the bottom portion, remaining in contact with the surface of the container and of the piston head for longer, losing a few temperature degrees);        at the starting points of the casting branches, where metal at the liquid state has greater fluidity,        at the mechanical interference points that could occur at the container openings (liquid metal loading inlet and casting channel starting points).        
In an attempt to obviating such disadvantage, a piston having a radial pin and a sealing band has been proposed, which at the step-wise split or interruption exhibits a seat suitable for receiving said pin.
Such solution however makes the assembly of the band on the piston more difficult and in any case requires separate means for the axial and angular locking of the band.